1. Field of the Invention
The present invention relates to a method for producing a bonded substrate. Particularly, the present invention relates to a method for producing a bonded substrate comprising a Si1-xGex (wherein x is a number satisfying 0<x≦1) film in which a biaxial strain has been introduced.
2. Description of Related Art
A strained silicon substrate in which a strain is induced in a silicon layer has been proposed in addition to a conventional bulk silicon wafer and silicon-on-insulator (SOI). The strained silicon substrate is one of means for increasing the carrier mobility in a semiconductor device.
In a typical method for producing the strained silicon substrate, a SiGe layer containing 15% to 20% germanium (molar concentration) is epitaxially grown on a silicon wafer, and silicon is epitaxially grown on the SiGe layer to obtain a strained silicon layer (see, for example, R. People, “Physics and applications of GexSi1-x/Si strained layer structures,” IEEE Journal of Quantum Electronics, QE-22, 1696 (1986)).
The method is characterized in that, since the lattice constant of the SiGe layer is larger than the lattice constant of the silicon layer, a biaxial strain is laterally introduced in the silicon layer epitaxially grown on the SiGe layer. Accordingly, the mobility of electrons is increased.
There is also a report that, by inducing a biaxial strain in a Ge film, the absorption coefficient at 1.55 μm in a wavelength band for optical communications is increased (see, for example, D. D. Cannon, J. F. Liu, Y. Ishikawa, K. Wada, D. T. Danielson, S. Jongthammanurak, J. Michel and L. C. Kimerling: Appl. Phys. Lett. 84, 906 (2004) and Y. Ishikawa, K. Wada, D. D. Cannon, J. F. Liu, H. C. Luan, and L. C. Kimerling: Appl. Phys. Lett. 82, 2044 (2003)).
In this method, Ge is directly epitaxially grown on a silicon wafer at 500° C. to 800° C. In the cooling process, a biaxial strain is introduced in the Ge film due to the difference in expansion coefficient between Si and Ge (the expansion coefficient of Si is 2.6×10−6/K, while the expansion coefficient of Ge is 5.9×10−6/K).
The strained Ge film is possibly used as a light receiving element or the like in, for example, an optical integrated circuit in the future, which can be regarded as one of the important applications thereof.